1) Field of the Invention
The present invention relates to ring network protection that speedily restore a ring network from a network failure by reducing the number of times of communication cut-off.
2) Description of the Related Art
Conventionally, a redundant network is constructed as a backbone network to avoid communication cut-off in the event of a failure. However, the redundant network has a disadvantage in that an undesirable loop occurs in a packet transmission path when a packet is to be transmitted. As a result, the packet may continue to circulate on the loop path.
To avoid occurrence of the loop in the packet transmission path, a spanning tree protocol (STP) technology is specified as a standard in Institute of Electrical and Electronic Engineers (IEEE) 802.1D. According to this STP technology, a part of a loop path is logically blocked, thereby preventing the loop packet transmission path even in the redundant network.
FIG. 22 is one example of a network based on the STP technology. In the STP technology, all nodes in a network exchange control information with one another, and set a root node. The network having loops is logically blocked to make the network loop-free, whereby a tree-structure network centering on the root node is constructed.
If a network failure occurs, a tree-structure network is re-constructed around the root node. Therefore, it takes a long time to restore the network to its original state. Thus, the disadvantage of the STP technology is that the network based on the STP technology is not suitable as the backbone network.
Under these circumstances, (S. Shah and M. Yip, “Extreme Networks' Ethernet Automatic Protection Switching (EAPS), Version 1”, [online], [searched on Feb. 24, 2004 (H16)], Internet <URL: http://www.watersprings.org/pub/id/draft-shah-extreme-eaps-03.txt>) discloses a technique of Ethernet Automatic Protection Switching (EAPS) (registered trademark). According to EAPS, a master node logically blocks one of the ports connected to a ring network, and if a ring network failure occurs, the logically blocked port is released, thereby promptly resuming packet communication.
However, the conventional art disclosed in the above document has the following disadvantages. If a failure-occurs in the ring network, communication is cut off. Besides, even after the network is restored from the network failure, communication cut-off occurs until each node learns a new communication path.
Specifically, after restoring from the failure in the network, the master node transmits path information to the other nodes, to inform the other nodes about a new network path. Thus, the communication cut-off occurs twice.